The invention relates to a method for the production of a printing form by the scanning point-by-point and line-by-line of an image pattern along successive cylindrical peripheral image lines and engraving cavities along corresponding cylindrical peripheral engraving lines.
Generally, an image pattern which is to be reproduced is mounted on a scanning or image cylinder which is rotated during the process. The image pattern is scanned helically by a scanning device such as an optical scanner which is continuously guided parallel to the axis of the image cylinder. The image pattern can contain half-tone images and/or characters. The tone value of the scanned image points determine the amount of the reflected light and the reflected light is detected and converted into image signals. In order to produce a photogravure screen on the engraving cylinder, a screen signal is superimposed over the image signals.
The engraving device includes an engraving member such as a mechanical engraver which uses an engraving needle as a cutting tool. The engraving member moves continuously along the rotating engraving cylinder.
The screen signal creates a vibrating lifting movement of the engraving needle and the image signals determine the penetration depth of the engraving needle into the surface of the engraving cylinder. Thus, a series of cavities of varying depth are produced along a helical line at the surface of the engraving cylinder.
In one prior art system, the image and engraving cylinders are each driven by a separate synchronous motor. Another known system uses a single synchronous motor to rotate the image and engraving cylinders which are rigidly connected axially to each other.
The engraving cylinder is used in a photogravure rotary machine for a printing process. The cylinder is inked and the cavities retain an amount of ink depending upon the respective cavity depth. During the printing process, ink is transferred from the cavities to a printing carrier and the tone-value of the printed surface is determined by the depth of the respective cavities which are disposed in a screen defined on the cylinder surface. For multi-color printing, cavities for each color are engraved on a single cylinder. In the photogravure rotary machine, a color print is produced by superimposing the printing from different engraving cylinders for the respective colors. In carrying out the process, inaccuracies arising due to alignment errors can occur and these errors have a disturbing influence on a person viewing the finished color print.
Alignment errors can arise during the engraving of the engraving cylinder because of different positions of the gravure on the engraving cylinder for a set of colors. Errors can also arise during the printing process itself if the synchronism between the engraving cylinder and the printing carrier is not precise. The accuracy of the synchronism in the photogravure rotary machine can be corrected through known control devices but no possibility exist to correct errors which originate due to the inaccurate positioning of the gravure on the engraving cylinders. If a high quality of reproduction is required, the gravure must have an accuracy of about 1/100 mm.
In order to avoid paper loss during the printing process, it is the usual practice to have the gravure continuous or almost continuous over the surface of the engraving cylinder. In the case of the reproduction scale of 1:1, it is customary to select an engraving cylinder having a periphery equal to the length of the image pattern to be reproduced. To carry this out, it is necessary to have engraving cylinders having different diameters depending upon the size of the image pattern. According to prior art methods, the scanning and engraving take place synchronously so that the image pattern must also encompass the image cylinder in order to avoid scanning gaps. This is achieved by having image and engraving cylinders with equal diameters.
In the case of printing magazines, only a relatively few engraving cylinders are needed. In a business concerned with printing packages and decorative labels and the like, it becomes necessary to have engraving cylinders of different diameters. Likewise, such businesses require a large stock of image cylinders having different diameters in correspondence to the engraving cylinders. Furthermore, for certain processes such as a repeating process in a peripheral direction, the image cylinder somewhat smaller than the engraving cylinder must be available.
The changes in the various image cylinders result in an increased cost during the production of printing forms. In addition, the maintenance of a stock of image cylinders of different sizes and the cost in time to transport and install different image cylinders add considerably to the so-called "set-up" time for the engraving.
For the aforementioned prior art method, the scanning and engraving members move continuously along the respective image and engraving cylinders and the scanning and engraving occur along helically lines around the respective cylinders. This process has the advantage of a very simple control over the rotation of the cylinders and the advancement of the scanning and engraving members operates relatively faultlessly as long as special engravings need not be carried out.
One type of special engraving is the repeating of characters in the peripheral and axial directions of the engraving cylinder. In the course of the repeating process, an image pattern is frequently identically recorded along the circumference of the engraving cylinder. In one known repeating process, if a character is to be repeated N times, then the circumferential length of the image pattern and the image cylinder must be the Nth portion of the circumference of the engraving cylinder and the image cylinder must be driven with N-fold speed of rotation of the engraving cylinder. The possible number of characters is limited by this process due to the high speeds of rotation and the small diameter for the image cylinders. Thus, this process has only limited use and is costly to carry out.
In another known repeating process, the image and engraving cylinders have the same diameters. The image pattern is scanned and the data there obtained is stored in a digital format. The gravure of several characters takes place thereby through repeated reading of the data from the storage along a helical line of the engraving cylinder. In this process, a system error arises which, in the case of individual characters along the circumference of the engraving cylinder, becomes apparent in an axial displacement of the information content of the scanned image pattern with respect to the engraved cavities defining the screen.
In this process, additional system errors are avoided by having the total number of cavities recorded during one rotation of the engraving cylinder integrally divisible by the number of characters along the circumference of the engraving cylinder.
In the case of helical recording, this requirement cannot be satisfied because of the photogravure screen in which the cavities of one cylindrical peripheral engraving line are positioned with respect to the adjacent engraving line so as to fill gaps. The number of engraved cavities per rotation of the engraving cylinder is selected so that during the helical engraving there is a remainder of 1/2 of a cavity for each rotation of the engraving cylinder.
In view of these system errors, it appears that a multi-color printing can only be executed faultlessly if the characters are engraved in a superimposed manner on the engraving cylinder for a set of color plates. In actual practice, this condition can be fulfilled only with a considerable expenditure.
In the case of the repeating process in an axial direction, several characters are engraved one next to the other in the axial direction through a multi-fold scanning of the image pattern.
Between successive scannings along successive cylindrical peripheral image lines, the scanning member must be moved in an axial direction with respect to the scanning cylinder until it has traveled from one end of the image pattern to the other end. The engraving process is interrupted during the time the scanning member is being advanced.
The turning points for the advancement of the scanning member must be accurately synchronized with the lateral boundaries of the image pattern. This synchronization can be adjusted accurately only for one point of the lateral boundary of the image pattern because after one rotation of the image cylinder the scanning member has become displaced in the axial direction by the amount of the pitch of the helical line. From this, an uncertainty of the adjustment results and this can lead to an alignment error.
From the aforementioned examples, it is apparent that for certain special engraving cases, the principle of the scanning and engraving along helical lines leads to errors which can considerably reduce the quality of the reproduction.
The instant invention endeavors to overcome these disadvantages by an improved engraving process.